1. Field of the Invention
The present invention provides a tool center point calibration apparatus and method for accurately determining appropriate tool center points for a robot to allow for proper tool alignment and correction of robot programs.
2. Prior Art
A tool center point is a six-dimensional vector defining the tool tip position of a robot tool with respect to a robot face plate. The tool center point is essential so that the robot knows precisely where the tool is located. Although a robot will know the precise location of the robot face plate at any given time, a robot may not precisely know the tool center point for a specific tool due to a variety of reasons. The tool center point for a specific tool of a given tool type will vary between individual tools. Consequently, prespecified tool center points are not entirely accurate for an individual tool. Additionally, the tool center point may change due to a crash or wearing of the tool. The robot requires precise location of the tool center point for appropriate operation.
Consequently, methods of determining the tool center point will provide means for correcting a robot program if the tool position changes due to a crash, tool change or other extraneous factors. Additionally, accurate measurement can provide for the correction of a robot program generated with the assumption of a perfect tool (i.e., a tool which exactly aligns with specifications).
A known method for determining a tool center point is physically measuring the location and orientation of the tool with respect to the robot face plate and entering the measured tool center point. Subsequent to entering the initially measured tool center point, the gun is swiveled about the position assumed to be the tool center point. If the tip of the tool moves, the assumed tool center point is adjusted. When the tip of the tool does not move when pivoted about the assumed tool center point, the appropriate tool center point has been achieved. This procedure takes approximately fifteen minutes for a skilled technician and has an accuracy of about .+-.8 mm. The method requires a skilled technician, a significant amount of down time for the robot and only offers minimal accuracy.
A system for updating tool center point values for tools has been devised which utilizes a fixed target or bull's-eye in a precise location. The robotic arm is moved to the known target location and the tool is adjusted until the tool tip is appropriately aligned with the target. This procedure again requires a skilled technician and takes approximately ten minutes. The accuracy of the resulting tool center point is within approximately .+-.2 mm. The difficulty with this solution is that it requires a skilled technician, is time-consuming and may result in tool damage.
Alternatively, the tool center point may be determined in a prior art method by removing the tool from the robot, moving the robot face plate to a fixed, known position, remounting the tool and taking the tips to the same known position. This removal of remounting of the tool can take twenty minutes with an accuracy of only .+-.3 mm.
A double beam calibration unit has been designed, specifically for the calibration of arc welding units. See the article "Robotic Arc Welding in a Flash" by the applicant in Robotics Today, Vol. 2, Number 4, Fourth Quarter 1989. The drawbacks of this system are that it requires a pair of orthogonal beams and is limited to specific types of tools.
The object of the present invention is to provide an efficient apparatus and method for determining the tool center point of robotic tools which is applicable to different types of robotic tools. Additionally, the object of the present invention is to provide a method and apparatus for calibrating spot welding guns for robotic tools.